1. Field of the Invention
The present invention relates to a data input apparatus, and more particularly, to a position coordinates signal detecting circuit of a sensor panel for sensing position coordinates at which contacts occur.
2. Description of the Related Art
As information terminals become smaller in size and larger in functional complexity, the types of input devices for providing data And information to the information terminals become more varied. For example, one input apparatus that is widely used for providing input commands or characters is a sensor panel coated with a resistance component. In such a device, characters and information are entered by pressing on the sensor panel at particular predefined locations. By detecting the position at which a contact is made on the panel, the system can determine the desired input. To that end, sensor panels typically include detection circuitry for detecting the locations of contacts.
In one example of such a position coordinates signal detecting circuit of a sensor panel, the position at which a contact occurs is detected by detecting the pairs of voltages at both ends of series resistances connected to the four edges of the sensor panel. The sensor panel resistances are driven by applying a driving signal of the same level through the connected resistors. Since the pairs of voltages on both sides of the contact point change according to the position of the contact point, it is possible to display the position of the point at which the contact occurred as coordinates by detecting the pairs of voltages at both ends of the resistors.
In conventional technology, four differential amplifiers are used for a position coordinates signal detecting circuit in order to separately amplify the pairs of voltages at the ends of the resistors connected to the respective edges. Also, variable resistors are generally used for the resistors connected to the respective edges of the sensor panel in order to correct internal impedance mismatching at the respective corners of the sensor panel.
In the position coordinates signal detecting circuit using four differential amplifiers, it is desirable that the characteristics of the differential amplifiers connected to the respective edges be identical to provide accurate position detection in the sensor panel of the location at which a contact occurred. That is, the respective differential amplifiers should generate the same level of output signals in a common mode and the amplification of the respective differential amplifiers should be the same. However, it is difficult to make the characteristics of the differential amplifiers identical, due to manufacturing processes. Accordingly, position coordinates of the point at which contacts occur may not be precisely detected due to the difference between the characteristics of the differential amplifiers.
FIG. 1 contains two waveforms labelled (a) and (b) which illustrate a signal output by differential amplifiers when there is no contact in the sensor panel i.e., in the common mode, under two conditions. For these waveforms, it is assumed that the impedances at the respective edges of the sensor panel are the same. Waveform (a) is an output waveform showing output characteristics of the four differential amplifiers in the common mode when the amplifier characteristics are different. Waveform (b) is an output waveform showing the output characteristics of the four differential amplifiers in the common mode when the amplifier characteristics are identical.
In a conventional position coordinates detecting circuit of a general sensor panel using four differential amplifiers, the output signals of the four differential amplifiers are multiplexed so that the output signal of the differential amplifier corresponding to a selecting signal is generated. For example, in one general conventional sensor panel, the output signals of the differential amplifiers connected to the left upper edge, the left lower edge, the right upper edge, and the right lower edge of the sensor panel are output from sections T1, T2, T3, and T4, respectively, in response to a selecting signal. When the output characteristics of the four differential amplifiers are identical in the common mode, signals having the same levels are output from the sections T1 through T4, in which the output signals of the respective differential amplifiers are selectively output as shown in waveform (b) of FIG. 1.
In actuality, because of fabrication process deviations, it is very difficult to achieve the desired result that the output characteristics of the differential amplifiers are identical in the common mode. Accordingly, the levels of the output signals of the four differential amplifiers selectively output from the sections T1 through T4 are different from each other as shown in waveform (a) of FIG. 1. Therefore, in a conventional position coordinates signal detecting circuit, it is difficult to precisely detect position coordinates of the point on the sensor panel at which contacts occur.
Also, in a conventional position coordinates signal detecting circuit, not only the output characteristics in the common mode, but also the amplifying degrees of the differential amplifiers may be different from each other due to the process deviation. In this case, since the signals output from the respective edges of the sensor panel are amplified to different amplifications, the position at which the contact actually occurred cannot be precisely detected.
To solve the above problems, it is an object of the present invention to provide a position coordinates signal detecting circuit of a sensor panel having a simplified structure and correctly sensing coordinates using only one differential amplifier.
Accordingly, to achieve the above and other objects, there is provided a position coordinates signal detecting circuit of a sensor panel for detecting a position coordinates signal of a contact point in a sensor panel coated with a resistance component. The circuit includes a plurality of e.g., first through fourth, resistors one end of each of which is connected to an edge of the sensor panel, e.g., the left upper-most edge, the left lower-most edge, the right upper-most edge, and the right lower-most edge of the sensor panel, respectively. The circuit also includes a plurality, e.g., first through fourth, driving signal generators respectively connected to the other side of the resistors for generating a driving signal so as to drive the sensor panel. The circuit also includes an analog switching means for receiving a plurality, e.g., first through fourth pairs of voltages respectively generated at both ends of the resistors, multiplexing the pairs of voltage in response to a plurality of e.g., first through fourth, selecting signals and providing a multiplexed signal. The circuit also includes a differential amplifier for inputting and amplifying the multiplexed signal.
The position coordinates detecting circuit of the sensor panel according to the present invention makes it possible to amplify the voltage difference between both ends of the variable resistor. Thus, the characteristics mismatching between amplifiers is reduced, and the position coordinates are detected accurately.